Tectonics of Europa 1
Title: Tectonics on Jupiter's icy moon Europa
Author: Kerry J. Cupit
Location:
Simon Fraser University
8888 University Drive
Burnaby, B.C.
Canada V5A 1S6
Contact email: [email protected]
Contact address:
10543 170A Street
Surrey, B.C.
Canada V4N 5H8
Running title: Tectonics of Europa
Keywords: convergent, divergent, water ice, ice shell, polar wander, tectonics, margins
Date submitted: Dec 5, 2008
Tectonics of Europa 2
(A) Introduction
(B) About Europa (C) Remote sensing (D) Evidence for tectonism (i) Dark bands (ii) Plates (iii) Chaotic terrain (iv) Strike-slip faults (v) Convergent margins (E) Tectonic models (i) Active lid versus stagnant lid (ii) Tidal stresses (iii) True polar wander (iv) Milankovich-like cycles (F) Similarities to Earth tectonics
(G) Conclusion
(H) References Cited (I) Figures
Tectonics of Europa 3
(A) Introduction
The ice surface of Europa appears unique in the solar system and exhibits small-scale
features that can be interpreted as plates with terrestrial tectonic similarities. By reconstructing
how these plates fit together in the past, insight into possible Europan tectonic processes can be
gleaned. Tectonic models can also be conceived and tested, based on the limited data returned
from this moon to date.
(B) About Europa
Europa is one of the four largest moons in orbit around Jupiter making up what are
known as the Galilean moons (Fig. 1). It is remarkable in that it is the only known planetary
body in the solar system to have a surface made up almost entirely of water ice floating atop a
possible subsurface liquid ocean (Showman and Malhotra, 1999; Gaidos and Nimmo, 2000;
O’Neill et al., 2007). This ice shell is one layer out of four that make up the composition of
Europa (Fig. 2); the icy surface, a theorized subsurface ocean, a rocky mantle and a nickel-iron
core.
The ice shell around Europa is suggested to be between 50 and 170km thick (O’Neill et
al., 2007) and crater counting methods have indicated that most of the surface is between 30 and
80 million years old (Figueredo and Greeley, 2004). The surface exhibits large patches of
different colours (Fig. 3), however the source of these colours is debated. Identified as sulfur
and silicates (McCord et al., 1998; McCord et al., 1999; Showman and Malhotra, 1999), if it is
assumed that the material associated with patchy colours were emplaced rather than converted
from in-situ non-coloured material, then the source is either a hypothesized ocean below
(McCord et al., 1998; McCord et al., 1999; Showman and Malhotra, 1999) or from sources
external to Europa (Cupit, 2007).
Tectonics of Europa 4
A subsurface liquid water ocean layer has been inferred through observations of a
dampened Jovian magnetic field around Europa (Gaidos and Nimmo, 2000) and has been
suggested to be up to 100km thick (O’Neill et al., 2007). Beneath this is a rocky silicate mantle
that Cupit (2007) suggested might have boundary layer interactions with the overlying
subsurface ocean in the form of black smokers due to tidal heating of the mantle. A nickel-iron
metallic core rests at the centre of Europa.
(C) Remote sensing
Unlike studying geology on Earth, planetary geology relies entirely upon remote
observations. It is typical for observations to be made from many thousands of kilometres away.
However, technology carried on new space probes has improved dramatically over forty years,
changing observation styles from the acquisition of low-resolution blurry dots (Fig. 4) to multi-
megapixel multi-panel images at very high resolution (Fig. 5). Advances in planetary spacecraft
navigation and survivability mean that probes can approach closer to Europa than before and
spend more time making more detailed observations. The earliest close-range data obtained
from Europa was a fly-by in 1973 by Pioneer 10 (Jet Propulsion Laboratory, 2003). Therefore,
knowledge about this moon has developed quickly since then and it is still an emergent field.
A number of probes have passed through or near the Jupiter system. However, only five
have made notable observations of Europa from which the tectonic discussions in this paper are
derived. They are the Pioneer 10 fly-by in 1973 (Jet Propulsion Laboratory, 2003), the Pioneer
11 fly-by a year later in 1974, the Voyager 1 and 2 fly-bys in 1979 (Hoppa et al., 1999a) and the
arrival of the Galileo orbiter in 1997 (Wikipedia contributors, 2008). The Galileo orbiter has
taken the highest-resolution photos of Europa to date, obtaining resolutions near 420 metres per
Tectonics of Europa 5
pixel (Sullivan et al., 1998). However, Galileo was only able to photograph 20-30% of Europa at
this resolution before its mission ended in 2003 (Schenk et al., 2008).
Remote sensing observations can be grouped into two types: optical and geophysical
observations. Optical observations are useful to most modern studies of Europan geology,
however a number of geophysical techniques are also used to characterize the environment
around Europa. Equipment used in geophysical observations includes magnetometers,
gravimeters, charged particle instruments, ultraviolet and infrared spectrometers, Geiger tube
telescopes, and plasma analyzers (Wikipedia contributors, 2008).
(D) Evidence for tectonism
Photos of the surface of Europa taken during flybys have been used in many studies to
identify features that may be indicative of tectonic processes operating on this moon. With an
average surface age of 10 million years (Showman and Malhotra, 1999), any geologic processes
would have been recently active or perhaps currently active, despite a noticeable decrease in
resurfacing activity over the last 30 to 80 million years (Figueredo and Greeley, 2004).
Very large scale fracturing (Fig. 6) and melting has been observed on Europa's surface
(Showman and Malhotra, 1999; Gaidos and Nimmo, 2000), which suggests that the forces
responsible for creating them are regional and operating on a near moon-wide scale. Showman
and Malhotra (1999) examine the case that tidal forces from Jupiter may be responsible, which
may also be a viable contributor to other tectonic-like features observed on Europa. Regardless
of tidal influence, geologic features on Europa have been grouped into craters, ridges, bands,
chaotic terrain and plains (Fig. 7). Craters are typically formed from bolide impacts and thus
Tectonics of Europa 6
aren't discussed further in the scope of Europan tectonics, since they rely on influences initially
external to the Jovian system.
(i) Dark bands
Voyager images from 1979 reveal dark, wedge shaped bands criss-crossing the surface
(Fig. 8) with roughly 50 to 100km spacing (Schenk and Seyfert, 1980) between brighter plains.
It is interesting to note that the bright plains bordering the dark bands appear to match very
closely with each other, with small amounts of rotation and translation applied (Sullivan et al.,
1998). The rotation is generally less than 10°, is in random orientations, and isn't necessarily
always present (Sullivan et al., 1998). Closer inspection of the dark bands reveal that they have
parallel lineaments and pit complexes in bilateral symmetry to a central lineament ridge pair
(Fig. 9a) (Sullivan et al., 1998). Figure 9b shows a brightness versus intensity graph of a dark
band ridge highlighting this bilateral symmetry. However, the study in which this was produced
did not take into account the illumination angle, resulting in lower troughs to the right of peaks
versus to the left of those same peaks, regardless of which side of the central lineament ridge pair
the data was obtained.
It can therefore be concluded that extrusion of material must be taking place at the centre
of these dark bands (Sullivan et al., 1998) under varying conditions to produce symmetrical
albedo characteristics. In order to produce the tens of metres of topography sometimes observed
along these central ridges, Sullivan et al (1998) assumed that any extruded material must have a
high viscosity or is rapidly quenched so that it is ultimately emplaced close to the central ridge.
Morphologically, these dark bands occur in linear, crescent shaped, and trapezoidal
shapes (Sullivan et al., 1998). Given the bilateral symmetry exhibited, these ridges likely
Tectonics of Europa 7
represent crustal extension and plate separation (Sullivan et al., 1998; Schenk et al., 2008) and
the newest crustal material present on Europa.
(ii) Plates
Bright, undeformed regions with sharply defined boundaries are interpreted as plates of
ice. Spaces between plates are usually dark (Sullivan et al., 1998). However, removing these
spaces and fitting plates back together to develop a tectonic history is relatively trivial. On larger
scales Euler poles must be used to account for plate rotation, but on scales of hundreds of
kilometres this correction need not be made (Sullivan et al., 1998). A sample plate
reconstruction can be seen in Figure 10. It is interesting to note that less than 1% of plate
material is missing in this reconstruction, which Sullivan et al (1998) suggests was converted to
dark material, covered by dark material, or somehow consumed in unobserved processes.
(iii) Chaotic terrain
Chaotic terrain covers about fifty percent of the surface of Europa (Greenberg, 2004) and
is characterized by the large-scale breakup and consumption of bright coloured plate material,
possibly through subsidence and/or burial (Sullivan et al., 1998). Pappalardo et al (1998)
suggests that features within chaotic terrain of the Conamara area of Europa are indicative of
thermally induced vertical displacement, hinting at convective cells occurring within the ice
shell.
Chaotic terrain represents one possible terrain type where plate material is consumed.
However, while dark bands are clearly evident of crustal extension, evidence for compensatory
crustal consumption is far subtler (Sullivan et al., 1998; Greenberg, 2004; Patterson et al., 2006).
Tectonics of Europa 8
Future studies may be able to elucidate a crustal surface area budget for each terrain type, but as
of now none yet exists.
(iv) Strike-slip faults
Close-up photos of the surface of Europa reveals increasingly compounded terrain types
(Sullivan et al., 1998). From these some researchers have identified a number of past and
currently active strike-slip faults (Fig. 11) (Hoppa et al., 1999b). One key result from the
research of Hoppa et al (1999b) is that the preferential direction of strike-slip faults are dictated
by which hemisphere they occur in. For the northern hemisphere, 80% of all identified strike-
slip faults are left-lateral. Whereas in the southern hemisphere, 95 to 100% of all strike-slip
faults are right-lateral. Europa's proximity to Jupiter means that it can be subjected to significant
tidal forces that act upon the ice shell (Hoppa et al., 1999b; Showman and Malhotra, 1999;
Gaidos and Nimmo, 2000). These forces appear to be the main mechanism whereby strike-slip
direction is determined in each hemisphere (Hoppa et al., 1999b).
(v) Convergent margins
As of 1998, there had been no studies performed that would indicate any presence of
convergent plate margins on Europa (Sullivan et al., 1998). However, studies conducted in 2003
and 2006 have since identified a few locations that are good convergent margin candidates
(Greenberg, 2004; Patterson et al., 2006). A good example of a proposed convergent margin is
shown in Figure 12. Greenberg (2004) made the observation that these margins do not exhibit
any structures similar to what would be expected on Earth, but instead have a subtle "muscle
tissue" appearance. The best method for identifying convergent margins of this type is to look
Tectonics of Europa 9
for two plates separated by a band and yet do not have edges that would match with each other if
they were brought together.
(E) Tectonic models
(i) Active lid verses stagnant lid
O’Neill et al (2007) suggested that planetary plate tectonic regimes could be classified as
"active lid" or "stagnant lid". Active lid tectonics involves downward moving cold lithosphere,
where stagnant lid tectonics involves lithosphere that is too strong to become incorporated into
the mantle. More specifically, stagnant lid tectonics occur when mantle convection stresses are
less than lithospheric stress, thus keeping intact a globally stable lithosphere. The average age of
the surface of Europa is 10 million years old (Showman and Malhotra, 1999). However, there
are no active large-scale resurfacing processes visible in observations to date. Therefore, O’Neill
et al (2007) theorize that Europa has had both active and stagnant lid periods throughout its
history resulting in occasional global resurfacing, similar to what has been suggested for
Venusian tectonics.
(ii) Tidal stresses
Tidal influences from Jupiter can impart significant stress to the surface of Europa
(Hoppa et al., 1999b) and to a lesser extent, so can the other Galilean moons (Figueredo and
Greeley, 2004). Gaidos and Nimmo (2000) postulate that tidal stresses can cause displacement
and friction along the edges of Europan strike-slip zones, producing warmer and more plastically
flowing ice. They calculate that displacement of 0.6m could sustain temperatures of 273K
within the fault zone, and that melted material would move upwards at a rate of tens of
Tectonics of Europa 10
centimetres per orbit possibly resulting in the formation of dark ridge structures over time. Dark
ridge structures with repeating curvilinear forms (Fig. 14) may have initially been formed as
cracks created by tidal forces over a period of a few Europan days (Hoppa et al., 1999a).
It appears that tidal forces are responsible for pushing nearby plates along strike-slip
faults, instead of stress release within a single plate (Hoppa et al., 1999b), therefore it may be
worthwhile to investigate this same phenomenon on Earth.
(iii) True polar wander
If the ice shell is decoupled from the silicate interior and varies in thickness latitudinally,
then it may be possible that polar wander is an appreciable outcome from regional stresses
(Schenk et al., 2008). No moon-wide features have been observed, but 0.3 to 1.5km deep arcuate
troughs hundreds of kilometres in length indicate about 80° of true polar wander through implied
stresses due to the reorientation of the surface relative to the moon's spin axis (Schenk et al.,
2008). These troughs appear to be at least be geographically related to the dark bands mentioned
previously, suggesting that many tectonic patterns on Europa may be related to true polar wander
(Schenk et al., 2008). Preferential directions of strike-slip faults noted in each hemisphere
support this hypothesis.
(iv) Milankovich-like cycles
Further research is required to investigate why the surface of Europa is relatively very
young compared to the ages of other planetary surfaces in the solar system. If Europa does
indeed undergo periodic global resurfacing, then various factors may contribute coincidentally to
either weaken lithospheric strength or strengthen the convective forces within Europa to the
Tectonics of Europa 11
point that active lid tectonics can take place. Figueredo and Greeley (2004) suggest that tidal
interactions between the four Galilean moons may lead to cyclic effects on Europa with a period
of approximately 100 million years. Hoppa et al (2001) identified an effect of nonsynchronous
rotation on the order of 250 thousand years that may also contribute to longer-term tectonic
processes.
(F) Similarities to Earth tectonics
Despite very different lithospheric mediums on Europa and Earth (water ice versus
silicates), there is evidence that similarities exist between the two planetary bodies. Heat
generated from the decay of radioactive elements is likely a contributing factor for heat transfer
in Europa (Showman and Malhotra, 1999), as it is also on Earth [textbook]. Additionally, there
is evidence for convergent, divergent and strike-slip margins between plates on Europa (Schenk
and Seyfert, 1980; Sullivan et al., 1998; Hoppa et al., 1999b; Figueredo and Greeley, 2004;
Greenberg, 2004; Patterson et al., 2006; Schenk et al., 2008). Divergent margins processes
expressed in the form of dark bands on Europa also bear a striking similarity to mid-oceanic
spreading ridges on Earth (Sullivan et al., 1998).
O’Neill et al (2007) notes that despite similar tectonic features on Europa, Earth is the
only known planetary body with active lid plate tectonics.
(G) Conclusion
Less than a third of the surface of Europa has been photographed at resolutions sufficient
for detailed plate tectonic studies to date, yet there have been numerous studies undertaken than
have produced abundant and valuable results. One obvious characteristic of this icy world is that
Tectonics of Europa 12
the plates are on a much smaller scale (10 to 50km wide) than the plates making up Earth's crust,
meaning that high-resolution photos are necessary for continued detailed studies of this moon.
The latest probe to visit Europa, Galileo, concluded its mission in 2003. Despite significant
scientific interest in Europa and the possibility that life might exist in its global subsurface liquid
water ocean, financial and political interests mean that it may be many years before closer
investigations are made of this world. In the meantime, data sets from Voyager and Galileo will
be used for many future studies of Europa, improving upon knowledge that has been
accumulated over 35 years since the days of the first Pioneer flybys.
The observation that tidal forces may play a role in Europan strike-slip faults suggests
that phenomenon observed on other worlds may be worthwhile to investigate closer to home, to
help fill in gaps in knowledge or suggest new lines of research on Earth. Planetary geology
therefore has implications not only for the theoretical understanding of other bodies in our solar
system, but for Earth-based processes as well.
Tectonics of Europa 13
(H) References Cited
Cupit, K., 2007, Introduction to exosedimentology (unpublished).
Figueredo, P.H., and Greeley, R., 2004, Resurfacing history of Europa from pole-to-pole
geological mapping: Icarus, v. 167, p. 287-312.
Gaidos, E.J., and Nimmo, F., 2000, Tectonics and water on Europa: Nature, v. 405, p. 637.
Greenberg, R., 2004, The evil twin of Agenor; tectonic convergence on Europa: Icarus, v. 167
(2), p. 313-319.
Hoppa, G.V., Tufts, B.R., Greenberg, R., and Geissler, P.E., 1999a, Formation of cycloidal
features on Europa: Science, v. 285, p. 1899-1902.
Hoppa, G., Tufts, B.R., Greenberg, R., and Geissler, P., 1999b, Strike-Slip Faults on Europa:
Global Shear Patterns Driven by Tidal Stress: Icarus, v. 141 (2), p. 287-298.
Hoppa, G.V., Tufts, B.R., Greenberg, R., Hurford, T.A., O'Brien, D.P., and Geissler, P.E., 2001,
Europa's rate of rotation derived from the tectonic sequence in the astypalaea region: Icarus, v.
153, p. 208-213.
Tectonics of Europa 14
Jet Propulsion Laboratory, NASA, 2003, Galileo [online] Available from
http://www2.jpl.nasa.gov/galileo/ [cited 22 Nov 2008].
Jet Propulsion Laboratory, NASA, 2008, Voyager [online] Available from
http://voyager.jpl.nasa.gov/ [cited 22 Nov 2008].
Lee, S., Zanolin, M., Thode, A.M., Pappalardo, R.T., and Makris, N.C., 2003, Probing europa's
interior with natural sound sources: Icarus, v. 165, p. 144-167.
McCord, T.B., Hansen, G.B., Fanale, F.P., Carlson, R.W., Matson, D.L., Johnson, T.V., Smythe,
W.D., Crowley, J.K., Martin, P.D., Ocampo, A., Hibbitts, C.A., Granahan, J.C. and Galileo Near
Infrared Mapping Spectrometer Team, United States (USA), 1998, Salts on Europa's surface
from the Galileo NIMS investigation: abstracts of papers submitted to the twenty-ninth lunar and
planetary science conference. Abstracts of Papers Submitted to the Lunar and Planetary Science
Conference, p. 29.
McCord, T.B., Hansen, G.B., Matson, D.L., Johnson, T.V., Crowley, J.K., Fanale, F.P., Carlson,
R.W., Smythe, W.D., Martin, P.D., Hibbitts, C.A., Granahan, J.C., Ocampo, A. and Galileo Near
Infrared Mapping Spectrometer Team, United States (USA), 1999, Evidence for hydrated salt
minerals on Europa's surface: lunar and planetary science, XXX; papers presented to the thirtieth
lunar and planetary science conference. Abstracts of Papers Submitted to the Lunar and
Planetary Science Conference, p. 30.
Tectonics of Europa 15
National Space Science Data Center, 2008, Photo gallery [online] Available from
http://nssdc.gsfc.nasa.gov/photo_gallery/ [cited 24 Nov 2008].
O'Neill, C., Jellinek, A.M., and Lenardic, A., 2007, Conditions for the onset of plate tectonics on
terrestrial planets and moons: Earth Planetary Science Letters, v. 261, p. 20-32.
Pappalardo, R. T. et al., 1998, Morphological evidence for solid-state convection in Europa’s ice
shell: Nature, v. 391, p. 365-368.
Patterson, G.W., Head, J.W., and Pappalardo, R.T., 2006, Plate motion on Europa and nonrigid
behavior of the icy lithosphere; the castalia macula region; faulting and fault-related processes on
planetary surfaces: Journal of Structural Geology, v. 28, p. 2237-2258.
Schenk, P.M., and Seyfert, C.K., 1980, Fault offsets and proposed plate motions for Europa: Eos,
v. 61, p. 286.
Schenk, P., Matsuyama, I., and Nimmo, F., 2008, True polar wander on Europa from global-
scale small-circle depressions: Nature, v. 453, p. 368-371.
Showman, A.P., and Malhotra, R., 1999, The Galilean satellites: Science, v. 286, p. 77.
Sullivan, R.J., Greeley, R., Homan, K., Klemaszewski, J.E., Belton, M.J.S., Carr, M.H.,
Chapman, C.R., Tufts, R., Head, J.W.,III, Pappalardo, R.T., Moore, J.M., Thomas, P., and
Tectonics of Europa 16
Galileo Imaging Team, United States (USA), 1998, Episodic plate separation and fracture infill
on the surface of Europa: Nature, v. 391, p. 371-373.
Wikipedia contributors, 2008, Galilean satellites [online] Available from
http://en.wikipedia.org/wiki/Galilean_moons [cited 12 Nov 2008].
Tectonics of Europa 17
(I) Figures
Figure 1: The four largest moons around Jupiter known as the Galilean moons, and Jupiter.
Sizes are to scale, however spatial relationships are not. Wikipedia contributors (2008).
Tectonics of Europa 18
Figure 2: Cutaway view of Europa showing four proposed layers: the 20-150km thick ice shell
(O’Neill et al., 2007), the subsurface ocean, the silicate mantle and the nickel-iron metallic core.
Wikipedia contributors (2008).
Figure 3: Global view of Europa, showing large-scale dark ridge structures, craters, and patchy
colours. Wikipedia contributors (2008).
Tectonics of Europa 19
Figure 4: One of the best images of Europa taken by Pioneer 10 during its flyby in 1973, with
161km/pixel resolution. Image at left is a colour composite, whereas the image at right is a
computer-enhanced version. Jet Propulsion Laboratory (2003).
Figure 5: Sample of a high-resolution photo taken from the Galileo space probe of Europa,
showing dark ridges, colour variations and a prominent crater. Image is 1240km across.
Wikipedia contributors (2008).
Tectonics of Europa 20
Figure 6: Dark ridges on a near global scale on the surface of Europa, possibly implying large-
scale fracturing and melting. National Space Science Data Center (2008).
Tectonics of Europa 21
Figure 7: Features on Europa are classified into one of five broad categories: craters, chaos
terrain, ridges, bands, and plains. Arrows highlight classified features in these images.
Figueredo and Greeley (2004).
Tectonics of Europa 22
Figure 8: Lower hemisphere image of Europa taken by Voyager, showing 50-100km separations
between dark bands on the surface. Sullivan et al (1998).
Tectonics of Europa 23
Figure 9: Close-up image of a dark ridge structure on Europa (a). Brightness versus distance
graph from A to A’ (b), showing central ridge pair and bilateral symmetry. Sullivan et al (1998).
Figure 10: Sample plate reconstruction. Colours are artificial and applied simply to aid
reconstruction. Sullivan et al (1998).
Tectonics of Europa 24
Figure 11: Examples of right-lateral strike slip motion (left panel), and left-lateral strike slip
motion (right panel). Hoppa et al (1999b).
Figure 12: Example of a possible convergent margin on Europa, as evidenced by the dark
circular feature being present on one side of a band and not the other, indicating some
consumption of material has taken place. Also note the “muscle tissue” appearance of the central
band. Greenberg (2004).
Tectonics of Europa 25
Figure 13: Curvilinear dark bands on the Europan surface. Possibly created through tidal
stresses, where each arc segment is created in one Europan rotation about Jupiter. Hoppa et al
(1999a).
